Capacity type pressure sensor and method of manufacturing the pressure sensor

ABSTRACT

A capacitive pressure sensor includes a first substrate ( 1 ), a first flat electrode ( 1   a ) formed on the first substrate ( 1 ), a pressure-sensing frame ( 4 ) surrounding the first flat electrode ( 1   a ) provided on the first substrate ( 1 ), second substrates ( 2, 3 ) connected to the pressure-sensing frame ( 4 ) opposedly to the first substrate ( 1 ) and forming a capacitance chamber ( 7 ) together with the first substrate ( 1 ) and the pressure-sensing frame ( 4 ), a stage ( 5 ) provided on the second substrates ( 2, 3 ) in the capacitance chamber ( 7 ), separated from the pressure-sensing frame ( 4 ), and opposed to and separated from the first flat electrode ( 1   a ), and a second flat electrode ( 2   a ) provided on the stage ( 5 ) and opposed to the first flat electrode ( 1   a ), wherein the pressure-sensing frame ( 4 ) deforms elastically according to a pressure applied to the first and second substrates ( 1, 2, 3 ).

TECHNICAL FIELD

[0001] The present invention relates to a capacitive pressure sensor fordetecting a change in capacitance (change in electrode-to-electrodedistance) based on the deformation of a capacitance chamber caused byapplication of a pressure from the outside, and a method ofmanufacturing the same.

BACKGROUND ART

[0002] In a conventional general capacitive pressure sensor, twoopposing electrodes are arranged in a capacitance chamber surrounded bya flexible diaphragm and a base. With this arrangement, a change inelectrode-to-electrode distance based on the elastic deformation of thediaphragm is detected as a change in capacitance, so that an externalpressure applied to the diaphragm is measured.

[0003] WO96/27123 (reference 1) proposes a pressure sensor based on anidea different from a conventional one. In the pressure sensor proposedby reference 1, a capacitance chamber is formed in an elastic substratemade of sapphire or the like, and two opposing electrodes are arrangedin the capacitance chamber. With this arrangement, the substrate itselfconstituting the capacitance chamber is compressed to deform uponapplication of a pressure. A change in electrode-to-electrode distanceaccompanying the deformation of the substrate is detected as a change incapacitance.

[0004]FIG. 9 shows the pressure sensor disclosed in reference 1. Asshown in FIG. 9, this pressure sensor is fabricated by bonding asapphire substrate 101 with a recess where a lower electrode 101 a andlead wire 101 b are formed and a sapphire substrate 102 with a recesswhere an upper electrode 102 a and lead wire 102 b are formed, such thatthe electrodes 101 a and 102 a oppose each other.

[0005] The upper electrode 102 a is formed on the substrate 102 thickerthan a diaphragm, as shown in FIG. 10. When an external pressure isapplied, the substrate 102 does not substantially deform, but asubstrate portion (to be referred to as a pressure-sensing frame 104hereinafter) constituting the side wall around the capacitance chamberis compressed to deform. When the pressure-sensing frame 104 deforms,the electrode-to-electrode distance changes, and a change in capacitanceis detected. The Young's modulus of sapphire is 30,000 kg/mm².

[0006] Accordingly, in this pressure sensor, when a pressure is applied,the lower electrode 101 a and upper electrode 102 a move close to eachother while being parallel to each other, and advantages are obtained,e.g., the linearity of the sensor output is maintained. Thepressure-sensing frame 104 is not easily compressed when compared to thediaphragm. Hence, this pressure sensor can be applied to a use under ahigh pressure, e.g., pressure measurement in an injection molder.

[0007] In the conventional example, however, when theelectrode-to-electrode distance is decreased in an attempt to improvethe measurement sensitivity, the pressure-sensing frame 104 is decreasedin its size in the longitudinal direction and is not compressed easily,which undesirably decreases the measurement sensitivity. Therefore, theconventional structure can be applied to only a use under a highpressure, and has a very narrow measurement range.

[0008] It is an object of the present invention to provide a capacitivepressure sensor in which the measurement sensitivity is improved.

[0009] It is another object of the present invention to provide acapacitive pressure sensor in which the measurement range can bewidened.

DISCLOSURE OF INVENTION

[0010] In order to achieve the above objects, a capacitive pressuresensor according to the present invention comprises a first substrate, afirst flat electrode formed on the first substrate, a pressure-sensingframe for surrounding the first flat electrode formed on the firstsubstrate, a second substrate bonded to the pressure-sensing frame andopposed to the first substrate, to form a capacitance chamber togetherwith the first substrate and the pressure-sensing frame, a stage formedon the second substrate in the capacitance chamber, separated from thepressure-sensing frame, and opposed to and separated from the first flatelectrode, and a second flat electrode formed on the stage and opposedto the first flat electrode, wherein the pressure-sensing frameelastically deforms according to a pressure applied to the first andsecond substrates.

[0011] A method of manufacturing a capacitive pressure sensor accordingto the present invention comprises the steps of forming a recess in afirst substrate and then forming a first flat electrode on the recess,forming a groove having a predetermined depth in one surface of anelastic second substrate to extend circumferentially, bonding a thirdsubstrate to one surface of the second substrate directly, polishing theother surface of the second substrate to expose the groove, thus forminga stage surrounded by the groove and forming a second flat electrode onthe stage, and bonding the first substrate and a pair structure of thesecond and third substrates to each other directly, to form acapacitance chamber where the first and second flat electrodes oppose.

[0012] With this arrangement, according to the present invention, thelength of the pressure-sensing frame which elastically deforms uponapplication of a pressure can be maintained sufficiently large. Sincethe upper electrode is formed on the stage, the distance between theupper and lower electrodes can be decreased. Thus, the measurementsensitivity and the measurement range can be further improved.

BRIEF DESCRIPTION OF DRAWINGS

[0013]FIG. 1 is an exploded perspective view of a capacitive pressuresensor showing the first embodiment of the present invention;

[0014]FIG. 2A is a sectional view taken along the line A-A′ of FIG. 1,and FIG. 2B is a sectional view taken along the line B-B′ of FIG. 1;

[0015]FIG. 3A is a plan view of the upper electrode shown in FIG. 1, andFIG. 3B is a sectional view taken along the line C-C′ of FIG. 3A;

[0016]FIG. 4A is a plan view of an upper electrode showing the secondembodiment of the present invention, and FIG. 4B is a sectional viewtaken along the line D-D′ of FIG. 4A;

[0017]FIG. 5A is a plan view of an upper electrode showing the thirdembodiment of the present invention, FIG. 5B is a plan view of a lowerelectrode corresponding to the upper electrode shown in FIG. 5A, FIG. 5Cis a sectional view taken along the line E-E′ of FIG. 5A, and FIG. 5D isa circuit diagram formed by the upper and lower electrodes shown in FIG.5C;

[0018]FIGS. 6A to 6E are sectional views taken along the line A-A′ toshow the steps in manufacturing the pressure sensor in FIG. 1;

[0019]FIGS. 7A to 7C are sectional views taken along the line B-B′ toshow the steps in manufacturing the pressure sensor in FIG. 1;

[0020]FIGS. 8A and 8B are plan and sectional views, respectively, of thepressure sensor for explaining its sensitivity;

[0021]FIG. 9 is an exploded perspective view of a conventionalcapacitive pressure sensor; and

[0022]FIG. 10 is a sectional view taken along the line F-F′ of FIG. 9.

BEST MODE OF CARRYING OUT THE INVENTION

[0023] The present invention will be described in detail with referenceto the drawings.

[0024] As shown in FIG. 1, a pressure sensor according to thisembodiment is formed of a sapphire lower substrate 1 in which arectangular lower electrode (stationary electrode) 1 a and a lead wire 1b are formed in a recess 7 a constituting a capacitance chamber 7 (FIGS.2A and 2B), a sapphire intermediate substrate 2 having a substantiallysquare upper electrode (movable electrode) 2 a (FIGS. 2A and 2B)arranged in the capacitance chamber 7 to oppose the lower electrode la,and a slab-like upper substrate 3 bonded to the intermediate substrate2. The lower surface of the intermediate substrate 2 is bonded to theupper surface of the lower substrate 1. The lower surface of the uppersubstrate 3 is bonded to the upper surface of the intermediate substrate2.

[0025] As shown in FIG. 2A, the upper electrode 2 a is formed on thelower surface of a region (to be referred to as a stage 5 hereinafter)partitioned by a groove 2 c formed in the intermediate substrate 2. Thelower electrode 1 a is formed on the bottom surface of the recess 7 a ofthe lower substrate 1. the groove 2 c is formed circumferentially like arectangular frame to correspond to the four sides of the capacitancechamber 7 (the recess 7 a of the lower substrate 1). A periphery 4 a ofthe intermediate substrate 2 which is separated from the stage 5 by thegroove 2 c and a side wall 4 b of the capacitance chamber 7 form arectangular frame-like pressure-sensing frame 4 which elasticallydeforms by an external pressure. As shown in FIGS. 2B and 3A, the upperelectrode 2 a is connected to an external device (not shown) through alead wire 2 b on a bridge 2 d formed in part of the groove 2 c.

[0026] The operation of the pressure sensor described above will bedescribed.

[0027] As shown in FIG. 2A, the groove 2 c separates the periphery 4 afrom the stage 5 where the upper electrode 2 a is formed. Hence, thepressure-sensing frame 4 which elastically deforms upon application ofthe pressure is long in the direction of pressure application (thedirection of compression). Hence, in this structure, a portion which issensitive to a pressure change can be maintained long while the distancebetween the electrodes 1 a and 2 a is kept narrow. Accordingly, thepressure sensor according to this embodiment can realize an improvementin measurement sensitivity and increase of the measurement range.

[0028] The bridge 2 d , which is formed in part of the groove 2 c inorder to extend the lead wire 2 b connected to the upper electrode 2 a ,is very small when compared to the entire size of the groove 2 c , asshown in FIGS. 3A and 3B. Hence, the bridge 2 d does not adverselyaffect the compression deformation of the pressure-sensing frame 4, anddoes not hinder pressure measurement.

[0029] An upper electrode 2 a and a lower electrode la according toother embodiments will be described with reference to FIGS. 4A and 4B,and FIGS. 5A to 5D.

[0030] In the second embodiment, as shown in FIG. 4A, a referenceelectrode 2 e is formed in the lower surface of an intermediatesubstrate 2 so as to surround an upper electrode 2 a on a stage 5through a groove 2 c . The reference electrode 2 e is used to measure areference capacitance together with a lower electrode la. In this case,the recess of the lower substrate 1, i.e., the circumference of acapacitance chamber 7 extends to the outside of the groove 2 c , and thereference electrode 2 e is arranged on the upper surface of the extendedportion to oppose the lower electrode 1 a.

[0031] The measurement result obtained with the movable electrode 2 acan be corrected by using the measurement result obtained with thereference electrode 2 e . When a pressure is applied, theelectrode-to-electrode distance between the reference electrode 2 e andlower electrode 1 a depends on a length β of the side wall of thecapacitance chamber 7, and that between the upper electrode 2 a andlower electrode 1 a depends on a length a of a pressure-sensing frame 4.For example, when α is set to almost satisfy α>100β, the measurementsensitivity of the lower electrode 1 a can be maintained sufficientlyhigh.

[0032] According to the third embodiment, as shown in FIGS. 5A to 5C,two lower electrodes 1 a-1 and 1 a-2 divided to oppose an upperelectrode 2 a are formed in the recess of a lower substrate 1. Theelectrically floating upper electrode 2 a, together with the lowerelectrodes 1 a-1 and 1 a-2, forms a series circuit of two capacitors, asshown in FIG. 5D. In this case, even when the upper electrode 2 a is inthe floating state, if the two lower electrodes 1 a-1 and 1 a-2 areprepared, a pressure change can be measured. According to thisembodiment, a bridge for the lead wire to be extended from the upperelectrode 2 a becomes unnecessary, and the manufacture becomes easierthan in the first embodiment.

[0033] A method of manufacturing the pressure sensor shown in FIG. 1will be described with reference to FIGS. 6A to 6E.

[0034] First, as shown in FIG. 6A, the upper surface of a base 2 as anintermediate substrate having a groove 2 c formed by a laser beam or thelike, and the lower surface of a base 3 for an upper substrate aremirror-polished, and the mirror surfaces of the two bases are bonded bydirect bonding in an atmosphere of about several hundred °C.Subsequently, as shown in FIG. 6B, the bottom of the base 2 is polisheduntil the groove 2 c is exposed. As a result, as shown in FIG. 6C, asubstrate unit 10 formed of an intermediate substrate 2 and lowersubstrate 1 is fabricated.

[0035] In the substrate unit 10 with this arrangement, the surface of agroove bottom X is a mirror-polished surface and is accordingly smooth.In contrast to this, when the base unit is fabricated by forming agroove in one base using a laser beam, it is almost impossible to form amirror-polished surface on the groove bottom X. If a substrate unit 10having a groove with a bottom surface which is not a mirror surface isused in a sensor, the stress acts nonuniformly, and the substrate tendsto be broken easily. In contrast to this, with the manufacturing methoddescribed above, the stress acts on the sensor substrate uniformly, andthe substrate can be prevented from being broken.

[0036] Subsequently, as shown in FIG. 6D, an upper electrode 2 a isformed on a stage 5 of the substrate unit 10 fabricated in FIG. 6C.Then, a recess and a lower electrode la are formed in and on a base 1for a lower substrate, and the substrate unit 10 and base 1 are bondedby direct bonding. In this case, direct bonding is performed at atemperature that will not damage (fuse) the electrodes 1 a and 2 a. As aresult, as shown in FIG. 6E, a pressure sensor having a pressure-sensingframe 4 which is long in the direction of pressure application isfabricated.

[0037] A pressure sensor in which the groove 2 c is formed in thesubstrate in a direction perpendicular to the pressure receiving surfacecan be fabricated in the above manner. In the pressure sensormanufactured in this manner, the region which is to be compressed uponapplication of a pressure can be increased while maintaining narrow thedistance between the lower electrode la and upper electrode 2 a.

[0038] A method of manufacturing the bridge 2 d will be described withreference to FIGS. 7A to 7C.

[0039] First, when a groove 2 c is to be formed in a base 2, the depthof the groove 2 c corresponding to the bridge 2 d is partly decreased,as shown in FIG. 7A. Then, as shown in FIG. 7B, a base 1 and the base 2are directly bonded to fabricate a substrate unit 10. Then, whenpolishing the lower portion of the substrate unit 10, a substrateportion corresponding to the bridge 2 d is left. A lead wire 2 b isformed on the bridge 2 d, as described above.

[0040] The sensitivity of the pressure sensor described above will bedescribed with reference to FIGS. 8A and 8B. A pressure sensor in whicha length a of the pressure-sensing frame 4 in the pressure-sensitivedirection satisfies a=2 (mm), an electrode-to-electrode distance bsatisfies b=0.1 (μm), a length c from a pressure-receiving surface 6 tothe bottom of the groove satisfies c=2 (mm), a width d of thepressure-sensing frame 4 satisfies d=0.4 (mm), a length e of thecapacitance chamber 7 in the x direction satisfies e=10 (mm), and alength f of the capacitance chamber in the y direction satisfies f=10(mm) is used. Assume that the Young's modulus of sapphire is 30,000kg/mm².

[0041] Under these conditions, (the area of the pressure-receivingsurface 6)=100 (mm²) and (the area of the pressure-sensing frame 4) (thearea of the bonding portion with the substrate 1)≠15.36 (mm²), and thepressure enlargement ratio is 100/15.36≠6.5. Assuming that the pressureapplied to the pressure-receiving surface 6 is 1 kg/cm² and that thedisplacement of the electrode-to-electrode distance b is Δb,Δb/2=1/30000×0.01×6.5 (mm), and thus Δb=43 (Å). Hence, when the basecapacitance is 100 pF, the sensitivity is 4.5 pF. Even when b=1 (μm),the sensitivity is 0.43 pF. In order that this structure functionseffectively, the thickness of the pressure-sensing frame 4 must belarger than the separation distance between the substrate 1 and thestage 5 on the substrate 3.

[0042] The above embodiment is described by way of a case that in whichsapphire is used as the material of the base, but the present inventionis not limited to this. For example, a single-crystal material such assilicon, glass, or diamond may be used. The reference electrode 2 e isnot an indispensable arrangement but may be added when necessary.Therefore, the present invention has the upper electrode 2 a and lowerelectrode la as the basic arrangement. The stage 5 may be formed on thelower electrode 1 a in place of on the upper electrode 2 a .

[0043] In the embodiment described above, the recess 7 a constitutingthe capacitance chamber 7 is formed in the lower substrate 1.Alternatively, a recess may be formed in the upper substrate which is tobe bonded to the lower substrate 1. In this case, the lower substrate 1suffices as far as it is a slab-like substrate.

[0044] In the embodiment described above, in order to mirror-finish thebottom of the groove 2 c, the pressure sensor is formed by bonding threesubstrates. When the groove bottom can be mirror-finished by laserprocessing or the like, the pressure sensor can be fabricated by bondingonly two substrates. In this case, this pressure sensor can be realizedby, e.g., inverting the substrate 2 shown in FIG. 6A and bonding it to alower substrate having a recess.

[0045] As has been described above, according to the present invention,the length of a region which is to be compressed to deform uponapplication of a pressure can be increased while maintaining large thedistance between the lower and upper electrodes, and an improvement inmeasurement sensitivity and increase of the measurement range can berealized.

1. A capacitive pressure sensor comprising a first substrate, a firstflat electrode formed on said first substrate, a pressure-sensing framefor surrounding said first flat electrode formed on said firstsubstrate, a second substrate bonded to said pressure-sensitive frameand opposed to said first substrate, to form a capacitance chambertogether with said first substrate and said pressure-sensing frame, astage formed on said second substrate in said capacitance chamber,separated from said pressure-sensing frame, and opposed to and separatedfrom said first flat electrode, and a second flat electrode formed onsaid stage and opposed to said first flat electrode, characterized inthat said pressure-sensing frame elastically deforms according to apressure applied to said first and second substrates.
 2. A capacitivepressure-sensor according to claim 1, characterized in that a regionopposing said first flat electrode is formed in part of saidpressure-sensing frame, and a reference electrode is formed in saidregion to oppose said first flat electrode.
 3. A capacitive pressuresensor according to claim 1, characterized in that said first and secondsubstrates are made of sapphire, silicon, glass, or diamond.
 4. Acapacitive pressure sensor according to claim 1, characterized bycomprising p1 a bridge for connecting part of said stage and part ofsaid pressure-sensing frame, and a lead wire which is formed on saidbridge and has one end connected to said second flat electrode and theother end extended to outside said capacitance chamber.
 5. A capacitivepressure sensor according to claim 1, characterized in that said firstflat electrode is formed of two flat electrodes arranged close to eachother, and a lead wire extended outside said capacitance chamber isconnected to each of said two flat electrodes, said two flat electrodesbeing formed to float electrically.
 6. A method of manufacturing acapacitive pressure sensor, characterized by comprising the steps offorming a recess in a first substrate and forming a first flat electrodeon the recess, forming a groove having a predetermined depth in onesurface of an elastic second substrate to extend circumferentially,bonding a third substrate to said one surface of the second substratedirectly, polishing the other surface of the second substrate to exposethe groove, thus forming a stage surrounded by the groove and forming asecond flat electrode on the stage, and bonding the first substrate anda pair structure of the second and third substrates to each otherdirectly, to form a capacitance chamber where the first and second flatelectrodes oppose.
 7. A method of manufacturing a capacitive pressuresensor according to claim 6, characterized by further comprising thesteps of forming part of the groove to be formed in the second substrateto have a depth smaller than those of other portions, and forming abridge in part of the groove by polishing, and forming, on the bridge, alead wire with one end to be connected to the upper electrode and theother end to be extended outside.